MATLAB: How to place a matrix of unsigned characters into a field in a MATLAB structure

charcharacterfieldlogical?MATLABmatrixmexmex filestructstructuretransferunsigned

How can I place a matrix of unsigned characters into a field in a MATLAB structure?

I am having difficulty in using a MEX-file to receive a 20-by-2 matrix of unsigned character values in C++ and transfer them to the same size matrix that is part of a structure in MATLAB.

Best Answer

To receive a matrix of unsigned characters into a field in a MATLAB structure, you can use our C API routines. These routines will copy the unsigned character matrix into the field of the MATLAB structure. The logical structure of an MATLAB structure is illustrated below:

 mxArray *(structure) #created using mxCreateStructureMatrix
     |
     |
       -- mxArray *(field1)  #created using mxCreate#####
     |
     |
      -- mxArray *(field2)  #created using mxCreate#####
     |
     |
      -- mxArray *(field3)  #created using mxCreate#####

An unsigned char is equivalent to an unsigned 8-bit integer, which you can create in your MEX-file using the mxCreateNumericArray routine:

field3 = mxCreateNumericMatrix(20, 2, mxUINT8_CLASS, mxREAL);
//The above says to create a real (as opposed to complex) 20 x 2 matrix of type "uint8".

You can now add this to the structure that was previously created with the mxCreateStructMatrix using the mxSetField routine.

 mxArray *structure;
 mxArray *field3;
 unsigned char *pdata;
 structure = mxCreateStructMatrix(1, 1, 3, {"field1", "field2","field3"});
 mxSetField(structure, 0, "field1", mxCreateDoubleScalar(20));
 mxSetField(structure, 0, "field2", mxCreateDoubleScalar(2));
 field3 = mxCreateNumericMatrix(20, 2, mxUINT8_CLASS, mxREAL);
 //returns the pointer to the data part of the MATLAB array
 pdata = mxGetData(field3);
 // Now copy actual data into the data part of field3
 for (int j = 0 ; j < 2 ; j++) {
    for (int i = 0 ; i < 20 ; i++) {
          pdata[20*j + i] = CppMatrix[i][j];
    }
 }
 mxSetField(structure, 0,"field3", field3);

Related Solutions

MATLAB: How to dynamically grow a structure in a MEX-file using the available MX routines

There is no direct API available to accomplish this. We recommended that you use the mxGetData, mxSetData, and the mxRealloc routines to dynamically grow your data. For example, the following MEX-function code will allow dynamic resizing of a structure using the available API. You should be able to copy this code to a file called 'growing.c' and compile this using the MEX command and run it in MATLAB:

/* growing.c */
/* Sample MEX function to dynamically grow a structure to create a vector structure */
#include "mex.h"
void mexFunction(int nlhs, mxArray * plhs[], int nrhs, const mxArray * prhs[])
{
      int m = 1;
      int n = 0;
      const char * fieldnames[] = {"field1", "field2"}; 
      int n_fields = sizeof(fieldnames)/sizeof(*fieldnames);
      int i,j,new_n;
      mxArray * pa;
      /* Start with a vector structure */
      pa = mxCreateStructMatrix(m, n, n_fields, fieldnames);
      /*  Grow the vector */
      new_n = n + 20;
      mxSetData(pa, mxRealloc(mxGetData(pa), new_n * n_fields *
      sizeof(mxArray *)));
      mxSetN(pa, new_n);
      /* NOTE: mxRealloc does not initialize additional memory */
      for(i=n; i < new_n; i++) {
              for(j=0; j < n_fields; j++) {
                      mxSetFieldByNumber(pa, i, j, NULL);
              }
      }
      plhs[0] = pa;
}

In MATLAB I see:

 >> mex growing.c
 >> growing
 ans = 
 1x20 struct array with fields:
     field1
     field2

NOTE: Please be aware that if you are not careful in using the mxSet* and mxGet* routines, your application may contain memory leaks or other issues.

MATLAB: How to destroy a structure array from a C MEX-file used in MATLAB

The documentation for MATLAB 7.6 (R2008a) has been updated to incorporate the relevant information. For previous product releases, read below for any possible workarounds:

Documentation on destroying structure arrays is not available in MATLAB 7.2 (R2006a). Here is additional information on destroying structure arrays in MATLAB:

If you create a structure matrix or array and use mxSetField or mxSetFieldByNumber to put another mxArray's data into the structure, it is not copied, rather the structure's field points to the other mxArray. When the structure is destroyed, it attempts to traverse down through itself and free all other data. Therefore, if the data array is destroyed and then the structure array is destroyed, the same memory is freed twice.

To workaround this issue, destroy the structure only as follows:

 mxArray *myDataOne; 
 mxArray *myDataTwo; 
 mxArray *aStruct; 
 const char *fields[] = { "one", "two" }; 
 myDataOne = mxCreateDoubleScalar(1.0); 
 myDataTwo = mxCreateDoubleScalar(2.0); 
 aStruct = mxCreateStructMatrix(1,1,2,fields); 
 mxSetField( aStruct, 0, "one", myDataOne ); 
 mxSetField( aStruct, 0, "two", myDataTwo ); 
 mxDestroyArray(aStruct);

Best Answer

Related Solutions

MATLAB: How to dynamically grow a structure in a MEX-file using the available MX routines

MATLAB: How to destroy a structure array from a C MEX-file used in MATLAB

Related Question